This invention relates to a friction device for use in car radios.
Car radios usually have a tuning slider which carries the cores of the variable-inductance coils of the local oscillator of the tuning device, thereby a tuning frequency change is obtained by correspondingly moving the tuning slider in its axial direction. The user may move the tuning slider by means of a convenient rotatable knob.
Furthermore, it is known to provide car radios, in addition to the conventional tuning knob, with keyboard type tuning devices in which each preselection key of a keyboard corresponds to a different transmitter station to which the key can be preset, whereby the radio can be tuned to said station by simply depressing this key. Such tuning devices generally employ a friction clutch, which is interposed between the tuning knob and the tuning slider and has to be released to disengage the manual knob control, when tuning is effected by using the keyboard in lieu of the knob.
A known type of such a releasable friction clutch employs a friction element with a high coefficient of friction, such as rubber, cork of the like. Therefore, the tuning devices of this known type of clutch have in general the disadvantage that the changes in tuning brought about by the rotation of the knob are non uniform, since the friction element of the clutch has a certain elasticity. Consequently, the tuning accuracy in these tuning devices is limited.
Furthermore, these known friction clutches are relatively cumbersome, because the friction element must be sufficiently large to transmit the torque necessary for moving the tuning slider.